1. Field of the Invention
This invention relates to controlling the brightness of light patterns created by a hologram. More specifically, this invention balances the brightness of a far field holographic light pattern and the clarity of a scene when viewed through a far field viewing device.
2. Background Information
Holograms of many different types have become commonplace in modern society. They are used as ornaments and as novelty items, as well as security devices on credit cards. A hologram is a pattern recorded on a substrate that provides a predetermined light diffraction effect.
There are many different types of holograms that are differentiated from one another by their optical properties and behavior. Most of the commonly seen holograms depend upon reflection of light from the hologram to the observer's eye. Less commonly seen are transmission type holograms wherein light passes through the hologram.
When an observer looks through a far field hologram at a scene that contains compact bright points of light, the observer sees holographic diffracted light patterns associated with each bright point location. We define this unique form of display holography as a far field viewing application. Far field viewing devices are made up of physical apertures (or frames) and far field holograms combined in a way designed for viewing a scene and superimposing holographic light patterns around each compact bright point of light in the scene.
Referring to FIG. 1, a far field viewing device containing of a far field hologram 10 mounted in a frame 12 is illustrated. The far field viewing device is placed in front of an observer's eye 14. The observer's eye 14 looks through far field hologram 10 mounted in frame 12 at a scene containing at least one bright compact source of light 16. Each point in the scene is viewed through a utilized hologram area 18. Schematic depictions of a tree and a star represent scene elements 20 that the observer wants to see in sharp focus.
Examples of far field viewing devices include the eyeglass device containing far field holograms as described in U.S. Pat. No. 5,546,198, as well as far field holograms mounted in windows. Ordinarily, a human observer looks through a far field device. Additionally, far field devices can also be incorporated into film-based or electronic image capture devices, such as still or motion cameras.
An example of an algorithm for calculating computer generated holograms is described by Gallagher and Liu. See N. C. Gallagher and B. Liu, “Method for Computing Kinoforms That Reduces Image Reconstruction Error” Applied Optics, v. 12, pp.2328-2335 (1973). The output of the algorithm is a set of numerical values. Each value corresponds to the desired complex transmittance at a different spatial location on the physical hologram. The resultant data set is used to drive any of a variety of fabrication methods which impose the desired transmittance values onto a physical substrate. There are a number of methods for producing a physical computer generated hologram from a set of date. These are summarized in the textbook MICROOPTICS [editor Hans P. Herzig, published by Taylor and Francis, London 1997] in chapters 4 and 5. An original hologram can be used as a master and copied or replicated using a variety of techniques as discussed in chapter of 7 of Herzig's MICROOPTICS.
Referring to FIG. 2, an idealized view of the overall scene as seen through an ideal far field viewing device is illustrated. The ideal view contains a well-focused representation of scene elements 220 in addition to a desired diffracted light pattern 222 produced by light diffracted by the far field hologram adjacent a bright compact source of light 216. In the example, the hologram has been tailored to diffract the light pattern in the form of letters spelling the word “NOEL”. FIG. 2 shows only one bright compact point of light to keep the illustration simple. In the case where many such sources of light are present, the desired diffraction pattern will surround each bright compact source of light.
A salient aspect of far field viewing applications that is different from most display hologram applications is that the observer is encouraged not to focus all of the attention on the holographic diffracted light pattern. Instead, the observer focuses on an overall scene in a unique combination with the holographic diffracted light patterns at each bright point source of light present in the scene. Accordingly, it is important for the viewing device to present a clear image of the scene while also presenting bright holographic light patterns.
It is also desirable for a far field viewing device to have a loose tolerance for the distance between the observer's eye and the hologram so that the viewer is not forced to maintain a particular position relative to the far field viewing device.
Additionally, it is desirable for the hologram in a far field viewing application to be capable of producing relatively large diffracted light patterns containing fine spatial detail.
The problem of balancing the clarity of the scene and the brightness of the holographic light patterns is not common in display holography. In most applications of display holography, the hologram is designed to diffract as much of the light as possible to create the brightest possible holographic reconstruction. Such a hologram is said to have high diffraction efficiency. The push in the industry is directed to design methods and fabrication processes that maximize the diffraction efficiency of display holograms since most applications of display holography call for maximum brightness in the holographic reconstruction.
Referring to FIG. 3, a view through a high diffraction efficiency far field hologram is illustrated. The scene elements appear as blurred images 324 when viewed through a far field transmission hologram having a high diffraction efficiency. FIG. 3 also shows that such a far field hologram also produces an undesired diffracted light pattern 326, symmetrically disposed about a bright compact light source 316 in the form of a mirror image of desired diffracted light pattern 322.
In contrast, our goal for far field viewing applications is to attain a diffraction efficiency that is often considerably less than the diffraction efficiency produced by standard methods for designing and fabricating holograms. When a highly efficient far field hologram is used in a far field viewing application, the diffracted light patterns are bright but the scene appears blurred. This effect on the view of the scene is much like looking through a light diffusing piece of shower glass, and it is undesirable since viewing, not obscuration, is desired. On the other hand, when the hologram has low diffraction efficiency the scene observed through the hologram appears well focused, but the holographic light patterns surrounding the point sources of light in the scene are not sufficiently bright.
Whereas the prior art provides no way to simultaneously maximize the scene clarity and the brightness of the holographic light patterns, we recognize that the diffraction efficiency of the hologram should be chosen to strike an optimum balance between the un-diffracted energy and the energy in the desired diffracted light pattern. The optimum diffraction efficiency can depend on the nature of the desired holographic pattern as well as the expected scene characteristics. Thus, flexible and simple control in achieving the desired diffraction efficiency of the hologram is needed.
One broad approach to the problem of reducing diffraction efficiency would be to start with an established method that produces high diffraction efficiency and to modify the approach to obtain reduced diffraction efficiency. The need for intentionally reducing diffraction efficiency of a far field hologram in a controlled manner has not been recognized in the prior art. In contract, we have made it a goal to increase the amount of un-diffracted light by reducing the amount of energy in the desired diffracted light pattern. Preferably, the modified process should not substantially increase the energy into undesired diffracted distributions that would distract from the desired diffracted pattern.
An unsatisfactory solution would be to modify standard hologram fabrication processes by adjusting process parameters to achieve the desired diffraction efficiency. In an amplitude hologram, it is possible to reduce the diffraction efficiency by reducing the transmittance contrast of the hologram. The transmittance contrast is a measure of the ratio of the highest transmittance to the lowest transmittance. Lowering the transmission contrast would in fact make the diffracted pattern weaker and improve the see-through performance of the hologram as desired. A significant drawback is that nonstandard processes would have to be developed to accomplish this. The use of nonstandard processes leads to increased costs and increased process variations.
In a binary phase hologram, it is possible to reduce the diffraction efficiency by changing the phase modulation depth. The phase modulation depth is a measure of the maximum optical path length difference between the two transmittance states in the hologram. As in the amplitude case, implementation of this solution would require processes that need tight control over transmittance contrast or phase modulation depth. Such processes are difficult to establish and maintain. These problems lead to increased costs and questionable repeatability, since non-standard fabrication procedures would be needed.
Additionally, a significant limitation of amplitude holograms and binary phase holograms is their restriction to Hermitian symmetric holographic light reconstruction patterns. Hermitian symmetry means that the desired reconstruction pattern is always accompanied by a copy of the pattern that is rotated by 180 degrees about the un-diffracted component. This undesired symmetric diffraction pattern in the form of a mirror image of the desired pattern is distracting in many cases. Furthermore, the undesired diffraction pattern takes up a large space that could otherwise be used to create larger and more complicated desired diffracted light patterns.
As discussed in our previous patent, U.S. Pat. No. 5,546,198, multilevel phase computer generated holograms (CGH's) can diffract light into asymmetric light patterns thus eliminating the distracting reversed diffracted copy and enabling a larger area for more complicated light patterns. In practice, such holograms are highly efficient and have poor see-through performance resulting in a severely blurred scene when used in a far field viewing device. The idea of decreasing diffraction efficiency by modifing process parameters is not an available option for multilevel phase CGH's. Unlike the case of binary CGH's, intentionally reducing the phase modulation to reduce diffraction efficiency of a multilevel CGH has serious undesirable consequences. As the phase modulation depth decreases, the diffraction efficiency does decrease but an additional diffracted pattern appears in the form of a reversed copy of the desired pattern. In practice, the strength of this reversed copy eliminates the advantage of multilevel phase holograms.
Referring to FIG. 4, a view of the scene through a multilevel phase CGH far field transmission hologram is illustrated. In this view, an undesired symmetric diffraction pattern has been eliminated so that only a desired diffraction pattern 422 is seen adjacent a bright compact light source 416. Elements of the natural scene are blurred as represented by blurred images 424.
Thus, an alternative form of the multilevel phase CGH is needed to balance see-through performance with the desired holographic reconstruction without introducing additional undesired diffracted light.
U.S. Pat. No. 5,210,625 and U.S. Pat. No. 5,278,008 disclose a multi-step process for modifying the diffraction efficiency of optically generated holograms without adjusting the contrast transmittance or the phase modulation depth over the whole hologram area. The disclosures of these patents are directed to beam splitting and redirecting holograms. They are silent regarding far field holograms, as well as information bearing holograms.
The process disclosed by the '625 and '008 Patents is not applicable to far field viewing devices. The disclosed aspect of introducing an unresolvable pattern of clear regions may be workable for image plane and Fresnel holograms when attention is focused at or near the plane of the hologram and may be useful for some beam redirection applications for which it is taught. However, the '625 and '008 disclosures do not recognize that the apertures defining the clear regions contribute to undesired diffracted light as well as un-diffracted light. The practical result is that the teachings of the '625 and '008 Patents cannot be applied to far field viewing applications because the small size of the unresolvable regions produces undesirable diffraction artifacts that compete with the desired reconstructions of far field holograms when bright compact sources of light are present in the scene.
Furthermore, the process of introducing unresolvable flat regions as the '625 and '008 Patents can introduce undesirable degradation in the see-through performance of holograms creating a blurred scene. The prior art concept of resolving the flat regions really loses meaning for holograms that are situated near the pupil of the eye as in the case of many far field viewing devices. Thus, different considerations are needed.
Moreover, the multi-step process disclosed by the '625 and '008 Patents is cumbersome and is not appropriate for computer generated holography.
What would be useful would be far field viewing devices incorporating holograms with diffraction efficiency adjusted to provide robust control over the balance between the clarity of the scene and the brightness of the holographic light pattern appearing at each bright point of light while minimizing undesired diffracted light patterns.